Tube type filters are typically used in industrial processes for purifying fluids in either the gaseous or liquid phase. Typical applications include, but are not limited to, coal gasifiers, fluidized bed combustors, smelters, and catalytic crackers.
As disclosed in U.S. Pat. No. 4,725,356, conventional tube-type filters generally comprise a tank or pressure vessel which is divided into an inlet portion and an outlet portion by a relatively rigid support member called a tube sheet. The tube sheet preferably forms a fluid tight seal between the inlet portion and the outlet portion and typically has a series of holes or apertures spaced from one another. Filter elements are mounted in the apertures of the tube sheet.
One type of conventional filter element is made of a porous ceramic material and is commonly referred to as a candle filter. Candle filters are particularly effective in removing particulates from high pressure, high temperature gases. A candle filter typically comprises a hollow, cylindrical tube that may have a narrow, elongated cross section, for example, a diameter in the range of 5-17 cm, and a length in the range of 0.1-3.0 m. The hollow tube typically has one end which is closed, one end which is open, and a porous, ceramic side wall. The porous side wall extends between the ends of the tube and defines an internal cavity that opens at the open end of the tube. The open end of the tube usually has a flange.
In a typical arrangement, one or more filter elements are arranged vertically inside the tank. Each filter element is inserted through an aperture in the tube sheet and is normally positioned such that the porous side wall and the closed end are located in the inlet portion of the tank while the flange of the tube is mounted to the tube sheet with the open end of the filter element communicating with the outlet portion of the tank. The flange has a larger diameter than the side wall of the filter element and allows the filter element to be located in and suspended by the tube sheet. A gasket material may be disposed between the flange and the tube sheet to form a seal and prevent bypass of unfiltered process fluid. A mechanism for clamping the filter elements against the tube sheet to secure the filter elements in position and maintain the seal is also provided.
With the filter elements mounted in place, a fluid, such as a gas laden with particulates, is introduced into the inlet portion of the tank. From the inlet portion, the gas passes through the porous wall of each filter element, where the particulates are removed from the gas, and into the internal cavity. The filtered gas then passes axially along the internal cavity of the filter elements and into the outlet portion of the tank through the open ends of the filter elements before exiting the tank through the outlet portion. As the fluid passes through the porous walls of the filter elements, particulates accumulate on the upstream side of the filter elements. With time, the particulates build up on the upstream side and form a particulate cake. The build-up of the particulate cake prevents fluid flow and, therefore, creates the need for the filter elements to be cleaned or replaced periodically.
One particularly effective method of cleaning the filter elements is by using a high pressure reverse gas flow. For example, cleaning may be accomplished by mounting a reverse cleaning fluid nozzle in close proximity to the open end of the filter element. A high pressure fluid (e.g., inert gas, air, or steam) is injected into the filter element in the reverse direction of normal fluid flow. The high pressure cleaning fluid is rapidly turned on and off to create a high pressure pulse in the back flow direction, dislodging the particulate cake formed on the upstream side of the filter elements.
Unfortunately, conventional tube-type filters are susceptible to a number of problems. Stress from improper clamping, mechanical vibrations, uneven contours on the filter surface or support, and thermal shock may result in damage to the filter element or leakage through the gasket. Back flow cleaning may introduce thermal shock to the filter elements if the high pressure reverse flow fluid is not at the same temperature as the filter elements. In addition, the use of a high pressure back flow cleaning pulse generates vibrations and mechanical stress on the filter elements. In candle filters, a common failure mode is cracking at the flange due to stress.
In addition, the tube sheet and the clamping mechanism are typically constructed using a metal alloy, while the filter element is typically construed of a ceramic material. Because ceramics typically have a much lower thermal coefficient of expansion than metal alloys, the ceramic filter elements and the metal tube sheet and clamping mechanism may expand at different rates due to temperature changes. These differing rates of thermal expansion can result in a reduction of the clamping force and leakage past the seal.